The present invention relates to a method for controlling the air-fuel ratio of an internal combustion engine such as an automotive engine and more particularly to a method of controlling the air-fuel ratio of an engine at a time when vaporized fuel in the fuel tank is purged into an engine.
As is well known, in the air-fuel control for a conventional engine, a learning control system has been introduced so as to correct a deviation of the air-fuel ratio derived from production variations or deteriorations in components such as an induction air flow sensor, a fuel injector and other components as quickly as possible and so as to keep the air-fuel ratio at a desired value even when the engine operating condition is largely changed. That is to say, at the previous running of the engine, a deviation of the centerline for the so-called LAMDA control coefficient is memorized on a map and at the present running, fuel injection amount is corrected by referring to the deviation memorized on the map, whereby the air-fuel ratio is controlled properly.
However, in recent years, restrictions have been placed on the releasing of vaporized fuel in the fuel tank to the atmosphere because of air pollution. To prevent the emission of the vaporized fuel, a so-called evaporative emission control system is widely used, wherein vaporized fuel in the fuel tank is adsorbed in a charcoal canister and then the vaporized fuel adsorbed therein is discharged into an induction system of an engine together with air. In this reference the discharging referred to as "canister purging" or "canister purge".
When a canister purge is carried out, commonly, the air-fuel ratio is deviated depending upon the amount of purged fuel being introduced into the engine. The deviation caused by the canister purge is learned as a deviation amount of the centerline for the LAMDA control coefficient and the map is rewritten by the above new learned data, whereby thereafter air-fuel ratio is adjusted as much as said deviation amount and is so controlled as to bring the deviated air-fuel ratio back to a desired value.
The problem is that when an engine is stopped and next started, adverse effects on engine startability and emissions are brought about, because the previously learned data are used even in an open loop control at an engine start and as a result, air-fuel ratio is substantially deviated.
To solve this problem, Japanese patent application laid open No.1988-129159 discloses a technology to control an opening/closing means which is disposed at a purging passageway for inducting vaporized fuel generated in the fuel tank into the induction system of an engine in such a way as the purging passageway is opened or closed with a specified interval based on the engine operating condition and to renew a learned value for the fuel amount in mixture gas based upon the detected air-fuel ratio when the purging passageway is closed, i.e., canister purge is inoperative.
However, in this prior art, a means for renewing a learned value only when a canister purge is stopped has a disadvantage that no correction is made to secular changes (deteriorations) or variations of components when canister purge is conducted, resulting in degraded controllability of the system.